Electroactive (e.g., piezoelectric) ceramics are intrinsically capable of generating only small strains. Various strain or electric field amplification devices, such as bimorphs or multi-layer actuators enhance the capabilities of such materials and enable the achievement of large linear displacements up to the millimeter level, with high force levels. In contrast, no similar electroactive devices are capable of generating large angular displacements (i.e. twists) with similar torque outputs.
For example, typical piezoelectric materials, such as lead zirconate titanate-based ceramics (PZTs) have a piezoelectric shear coefficient d.sub.15 in the range of 300 to 700 pm/V. The shear strain or twist angle produced by use of these materials is on the order of 0.02.degree., when a material having a d.sub.15 coefficient of 700 pm/V and an electric field E of 5 kV/cm is used. Such an angular displacement is too small for most practical applications.
Fuda et al. in "Piezoelectric Torsional Actuator", Ferroelectrics, 1994, Volume 160, pages 323-330, describe a piezoelectric torsional actuator which utilizes a single ceramic cylinder comprised of a PZT material. The key feature of the cylindrical actuator is an interdigital electrode network on the outer surface of the cylinder wherein the interdigitated electrodes are oriented at an angle of 45.degree. from the length axis of the cylinder. When the cylinder wall thickness and the ratio between electrode width and pitch are kept constant, Fuda et al. report that the rotational angle is approximately proportional to the electric field. Fuda et al. further report that the rotational angle of such a PZT cylindrical construct is inversely proportional to the diameter of the piezoelectric cylinder.
Because the torsional movement of the Fuda et al. piezoelectric cylinder makes use of the combined effects of the d.sub.33 and the d.sub.31 piezoelectric coefficients, the available torsional strain is small (on the order of microradians).
Currently, there is a need for torsional actuators for control of local and global twist in helicopter rotor blades, as control surface actuators for adaptive wings and in further air-moving applications such as turbo prop blades and unducted fan engine control mechanisms. There is thus a need for a solid state torsional actuator which can achieve substantial angular rotational displacements with relatively high torque values.
Accordingly, it is an object of this invention to provide an improved torsional actuator which makes use of electroactive materials.
It is another object of this invention to provide an improved torsional actuator that enables high torque angular displacements to be achieved.
It is a further object of this invention to provide an improved torsional actuator that makes use of the d.sub.15 piezoelectric shear coefficient, since it is known that the d.sub.15 coefficient is greater than either of the d.sub.31 or d.sub.33 piezoelectric coefficients.